1. Field of the Invention
This invention relates to elastomeric products reinforced with glass fabric or cords, and more particularly, to a method for treating the glass cords to eliminate the occurrence of cord blows in the glass reinforced elastomer.
The term "glass fibers", as used herein, refers to fibers formed, for example, by the rapid attenuation of hundreds of streams of molten glass, and to strands formed when such glass fiber filaments are gathered together. Generally, the term "glass cords" refers to the product formed by plying and/or twisting a number of glass strands together. Many elastomeric articles, especially those made from rubber, such as pneumatic tires, conveyor belts, power transmission belts, and the like are reinforced with glass cords or fabric which serve to increase the tensile strength of the article and minimize distortion under service conditions.
While the present invention can be used advantageously with any glass cord-reinforced elastomeric article, it is especially adapted for use in making pneumatic tires and will be described in connection therewith. Glass cords are most frequently used in the breaker skim of pneumatic tires, although glass cords can also be used in the carcass plies. The breaker is made by calendering the glass cords with the rubber compound.
Glass cords for use in tires are normally treated before calendering with an adhesive system containing resorcinol-formaldehyde resin and rubber latex (such as vinyl pyridine). Although the conventional dipped-glass fabric has performed satisfactorily in tires, it has been found that the durability of the cord is improved by an increase in the amount of dip on the cord, yielding what is referred to as a "high dip pickup" (DPU) glass fabric. Whereas the conventional glass fabric comprises about 15% to about 18% by weight of the dip, the high DPU glass fabric comprises about 30% by weight of the dip. The high DPU glass fabric has greater resistance to fracturing and powdering of the individual fibers during the normal flexing of the cord. Adhesion, and tensile strength are similarly improved.
It was found however, that tires using the high DPU glass fabric, as opposed to the conventional glass fabric, showed an increase tendency toward cord blows (loss of bundle integrity), which can be identified by a void between the cord and the rubber, and extending along the cord. The cord blows are most frequently found at the breaker edge, in the thicker shoulder portion of the tire where it takes longest to cure the rubber compound. The result may be a high speed failure and tread loss after a period of operation, as the air in the void around the cord heats, expands and propagates the blow circumferentially around the tire.
It is believed that this tendency for the cord to blow is related to the use of the high DPU glass fabric, in that it contains more unsaturated polymer which must be cured. This additional amount of polymer in the dip takes sulfur from the adjacent rubber compound, i.e., the curatives migrate from the curative-rich ply skim to the curative-deficient glass dip. The result is that the glass dip and the majority of the rubber compound are cured, but the compound immediately adjacent each cord is not fully cured or "compacted", so that when the curing mold pressure is released at the end of the cure cycle, gas trapped within the cord can expand, forcing the adjacent rubber compound out away from the cord and creating the void or "cord blow". While this is believed to be the mechanism by which a cord blow occurs, it should be clearly understood that the operation of the present invention is in no way dependent upon this particular mechanism or any of its specific steps.
2. Description of the Prior Art
One proposed solution to the problem of cord blows was the addition of more cure accelerator to the rubber compound. Although this additional accelerator enabled the rubber compound immediately adjacent each glass cord to cure within the normal curing cycle, it was found that the remainder of the skim compound cured prematurely, scorching the compound during calendering or other process operations.
The problem of cord blows with the high DPU glass fabric was temporarily overcome, sufficiently to enable continued use of the high DPU fabric, by increasing the curing cycle for the tire, for example, from about 14 minutes to about 15 minutes. The result was a decrease of about 7% in curing capacity, i.e., the number of tires having high DPU glass fabric which could be cured.